Help for a rook...

Welcome and don't worry, we all have to start somewhere.

First thing for you to know is that pre/pro stands for Preamp/Processor. The "preamp" pertains to audio and the "processor" pertains to audio & video processing. Your Denon AVR2112 is a pre/pro coupled with a multi channel power amp all in one box.

Second, to build your 5.1 system, you need to add a center channel and then a sub as you outline above. You will now have a 3.1 system. Finally you can consider surrounds to make it a 5.1 or 7.1 system.

Last, and hate to break it you, is your question about about adding amps for your main speakers. You can't do it with your current Denon AVR. It has no L/R pre-outs (stands for Preamp/Outputs). You are stuck using your Denons internal amps. The only way to get more and higher power into your B&W's is to ditch the Denon 2112 and get an AVR with a full set of pre-outs or go the separates route - a pre/pro and power amp in two separate cases. Hope this helps and good luck.

You should map out your upgrade path in steps... Each step should measurably improve your sound... (that juice keeps you going in the hobby)

I would get the matching center to your B&Ws,
Then I would make a decision on your sub.... do you match your sub quality to your three front speakers... OR do you think of the sub as a once in 10 year purchase and stretch for a keeper sub... that will shortly match upgraded LR B and W speakers. (You could also make the same decision on your center... buy an upgrade BW center and eventually complement it with better LR mains)...

The rear speakers will be situational based on your room and how you would mount and position them.... and should match the front three giving you a solid 5.1 system... with a really tight base.

Your next move would be to consider your digital front end versus your power and processing in your receiver.

At that point, you can decide to add more features to your digital front end based on you interest in storing media on a media server, spin all kinds of discs etc.... Something like an Oppo player... or a DAC-Preamp etc...

Or, you could upgrade the receiver with a newer more powerful receiver or improve things even more with separate units. Pre/Pro's and multichannel amps.

You ultimately will want to match your speakers to your amps and the room and your tastes and budget.... based on how much two channel music you listen to versus how much home theater you play.

Thanks for the input guys!